A known process for fabricating a plurality of thin film devices on a ceramic substrate, wherein the thin film devices include alpha tantalum capacitors, involves the initial steps of selectively glazing the substrate in proposed capacitor areas, sputtering a tantalum film on the substrate, thermally oxidizing the tantalum film to form a tantalum pentoxide underlay, and then sputtering an alpha tantalum (nitrogen-doped) film over the underlay. The alpha tantalum film then is selectively etched to produce capacitor base electrodes, initial layers of associated contact pads, and a system of anodization bus bars. Portions of the capacitor base electrodes then are anodized to form capacitor dielectrics, upon which upper capacitor electrodes subsequently are formed by known deposition and etching techniques.
More specifically, a plurality of the thin film circuit devices are formed on a substrate simultaneously and the substrate subsequently is separated along preselected lines to produce the individual thin film circuit devices. Further, the sputtering of the underlay and the alpha tantalum films on the substrate generally is accomplished in a sputtering chamber simultaneously with other substrates also disposed in the sputtering chamber.
One sputtering system which has been used for this purpose includes a cylindrical sputtering chamber having a rotatable carrousel for conveying substrate holders, each having a pair of the substrates mounted therein, around a circular target cathode of the tantalum material to be sputtered onto the substrates. In this apparatus, the circular target was fixedly mounted in the cylindrical sputtering chamber in encircling relationship to an annular-shaped combination gas distribution-target cooling cell. The gas distribution-target cooling cell was of split, essentially annular construction to permit the cell to be expanded radially outward and clamped against an interior surface of the target. More specifically, the gas distribution-target cooling cell was constructed with a system of interior passageways for carrying cooling water continuously through the cooling cell for target cooling purposes. The cooling cell also included a system of interior passageways for carrying a sputtering gas, consisting of a mixture of argon gas and a nitrogen doping gas, through the cell to an array of gas exit openings in an outer wall of the cooling cell. The gas exit openings in the outer wall of the cooling cell were aligned with respective small (1/32") apertures which had been drilled through the target, to permit the argon-nitrogen sputtering gas mixture to feed through the target into a sputtering area in the sputtering chamber between the target and the carrousel upon which the substrates were mounted. Gas diffuser devices also have been mounted in the drilled apertures, to spread the gas mixture emanating from the apertures over an outer surface of the target.
In the sputtering of the alpha tantalum film on the substrate as above described, in order for the film to be of acceptable quality for use in fabricating thin film capacitors, it is essential that the amount of interstitial contamination within the film be kept within certain preselected limits. It also is necessary that the nitrogen content in the alpha tantalum film be such that the film has a temperature coefficient of resistance (TCR) within a preselected range. Further, for satisfactory results the nitrogen content of the alpha tantalum film must be uniform throughout the film within preselected limits. In the prior known sputtering system discussed above, this required that upper and lower outer peripheral edges of the combination gas distribution-target cooling cell be fitted within the cylindrical target in tight-fitting sealed relationship with upper and lower interior peripheral edges of the target, to preclude leakage of the sputtering gas mixture from between the cell and the target upper and lower edges so as to produce areas in certain portions of the substrates which were excessively rich in nitrogen.
The attaining of adequate leak-proof seals between the upper and lower peripheral edges of the target and the combination gas distribution-target cooling cell, however, was difficult for various reasons. For example, it is difficult to fabricate the cylindrical target in the form of a true cylinder; rather, the target frequently is slightly out-of-round or has a slightly frusto-conical construction. Thus, the gas distribution-target cooling cell frequently could not be expanded into tight-fitting sealed engagement with the target. Further, since the target and the gas distribution-cathode cooling cell were assembled together within the sputtering chamber from the top thereof, access to the lower edges of the target and the cooling cell to determine whether they were in tight-fitting relationship was extremely difficult.
Accordingly, in preparing for an alpha tantalum sputtering operation the tantalum target initially was "cleaned up" by checking for vacuum leaks, pumping the sputtering chamber down to a high vacuum, and then initiating "bakeout" of the system. More specifically, tantalum was initially sputtered onto "clean-up" substrates for several hours at a high power level without nitrogen doping, to sputter-clean the target surface, and to heat the sputtering chamber walls and chamber interior fixturing so as to "out gas" any residual gas or organic contamination, which was then pumped away. The freshly deposited tantalum on the chamber interior surfaces and fixturing, and on the "clean-up" substrates, also facilitated "cleaning up" by causing "gettering" of any residual gases such as nitrogen or oxygen.
After the system was ready for production from a "clean" standpoint, the argon/nitrogen sputtering gas mixture was introduced into the system at a normal operating rate through the combination distribution gas-target cooling cell and the apertures in the target, to check for leaks between the upper and lower edges of the cell and the target which would produce nitrogen-rich alpha tantalum film having unacceptable TCR values. Since the sputtering system was in a closed "clean" operating condition, this was accomplished by sputtering tantalum for a preselected time period (several minutes) onto sets of sample substrates mounted in holders in respective spaced stationary positions about the interior of the sputtering chamber, such as four positions on the order of 90.degree. apart, preferably in independent separate sputtering operations. For example, a first one of the substrate holders was loaded into a position on the sputtering chamber carrousel, indexed into a respective one of the spaced positions in the sputtering chamber, and then subjected to sputtering for the preselected time period while in a stationary condition. After being sputtered, the first set of sample substrates was unloaded from the carrousel and the next set of sample substrates was loaded into the carrousel and sputtered in the same manner in its respective position in the sputtering chamber. After all of the sets of sample substrates had been sputtered in this manner, the TCR of the substrates in each holder was measured. A range in the TCR measurements in excess of a preselected value indicated that an unacceptable gas leak condition existed between one or both of the upper and lower edges of the gas distribution-target cooling cell and the target, and that the system would probably not produce acceptable parts. It then was necessary to shut the system down, open the sputtering chamber, and remount the target to the gas distribution-target cooling cell. The sputtering chamber then was re-closed and the above described "clean-up" and gas-leak detection operations were repeated, with a significant loss in production time and in the sputtering life of the target.
In addition to producing alpha tantalum films with nonuniform nitrogen doping as above described, the inability to mount the essentially annular combination gas distribution-target cooling cell in firm mating engagement with the interior of the target reduced the effectivenss of the cell in cooling the target during a sputtering operation. Further, since a large part of the cell was occupied by internal gas passageways, the space available within the cell for cooling water channels was reduced, thus limiting the target cooling capacity of the cell. As a result, the target tended to overheat during the sputtering operation, particularly if sputtering was carried out at too rapid a rate, thus limiting the production output of the apparatus. The pre-drilling of the gas feed apertures in the target, and the mounting of the gas diffusers in the openings, also was time consuming and reduced the amount of target material available for sputtering.
Accordingly, a purpose of this invention is to provide a new and improved simplified system for sputtering a film of uniformly doped material onto a substrate wherein the system also produces more efficient cooling of a target material being sputtered so as to enable production by the system to be increased.